Biomedical subjects

Vasily L Yarnykh

Publications and source records attributed to Vasily L Yarnykh.

12 recordsLinked to original sources

PubMed2007

Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field.

A new method has been developed for fast image-based measurements of the transmitted radiofrequency (RF) field. The method employs an actual flip-angle imaging (AFI) pulse sequence that consists of two identical RF pulses followed by two delays of different duration (TR(1) < TR(2)). After each pulse, a gradient-echo (GRE) signal is acquired. It has been shown theoretically and experimentally that if delays TR(1) and TR(2) are sufficiently short and the transverse magnetization is completely spoiled, the ratio r = S(2)/S(1) of signal intensities S(1) and S(2), acquired at the beginning of the time intervals TR(1) and TR(2), depends on the flip angle (FA) of applied pulses as r = (1 + n * cos(FA))/(n + cos(FA)), where n = TR(2)/TR(1). The method allows fast 3D implementation and provides accurate B(1) measurements that are highly insensitive to T(1). The unique feature of the AFI method is that it uses a pulsed steady-state signal acquisition. This overcomes the limitation of previous methods that required long relaxation delays between sequence repetitions. The method has been shown to be useful for time-efficient whole-body B(1) mapping and correction of T(1) maps obtained using a variable FA technique in the presence of nonuniform RF excitation.

Adult↗

PubMed2006

Multicontrast black-blood MRI of carotid arteries: comparison between 1.5 and 3 tesla magnetic field strengths.

PURPOSE: To compare black-blood multicontrast carotid imaging at 3T and 1.5T and assess compatibility between morphological measurements of carotid arteries at 1.5T and 3T. MATERIALS AND METHODS: Five healthy subjects and two atherosclerosis patients were scanned in 1.5T and 3T scanners with a similar protocol providing transverse T1-, T2-, and proton density (PD)-weighted black-blood images using a fast spin-echo sequence with single- (T1-weighted) or multislice (PD-/T2-weighted) double inversion recovery (DIR) preparation. Wall and lumen signal-to-noise ratio (SNR) and wall/lumen contrast-to-noise ratio (CNR) were compared in 44 artery cross-sections by paired t-test. Interscanner variability of the lumen area (LA), wall area (WA), and mean wall thickness (MWT) was assessed using Bland-Altman analysis. RESULTS: Wall SNR and lumen/wall CNR significantly increased (P < 0.0001) at 3T with a 1.5-fold gain for T1-weighted images and a 1.7/1.8-fold gain for PD-/T2-weighted images. Lumen SNR did not differ for single-slice DIR T1-weighted images (P = 0.2), but was larger at 3T for multislice DIR PD-/T2-weighted images (P = 0.01/0.03). The LA, WA, and MWT demonstrated good agreement with no significant bias (P 0.5), a coefficient of variation (CV) of < 10%, and intraclass correlation coefficient (ICC) of > 0.95. CONCLUSION: This study demonstrated significant improvement in SNR, CNR, and image quality for high- resolution black-blood imaging of carotid arteries at 3T. Morphologic measurements are compatible between 1.5T and 3T.

Adult↗

PubMed2006

Intra- and interreader reproducibility of magnetic resonance imaging for quantifying the lipid-rich necrotic core is improved with gadolinium contrast enhancement.

PURPOSE: To test the hypothesis that intra- and interreader reproducibility for measuring the lipid-rich necrotic core (LR-NC) size is significantly improved with gadolinium (Gd) contrast-enhanced magnetic resonance imaging (CEMRI) compared to non-CEMRI. MATERIALS AND METHODS: Thirty-seven individuals with >50% carotid artery stenosis underwent carotid MRI at 1.5T (pre- and postcontrast T1-weighted (T1W), T2-weighted (T2W), proton density-weighted (PDW), and three-dimensional time-of-flight (TOF) sequences). Two independent readers measured the mean area of the LR-NC from the precontrast images only, followed by a second measurement using the additional postcontrast images. One reader repeated the measurements after an interval of five months. Intra- and interreader reproducibility was analyzed by means of the intraclass correlation coefficient (ICC), coefficient of variation (CV), and standard deviation (SD). RESULTS: The CV decreased from 33.7% to 8.8% for intrareader measurements of the LR-NC, and from 33.5% to 17.6% for interreader measurements. The SD was significantly smaller with CEMRI than with non-CEMRI (P = 0.003 and P = 0.006, respectively). The ICC increased from 0.94 to 0.99 and from 0.85 to 0.93 for the intra- and interreader measurements, respectively. CONCLUSION: Reader reproducibility for in vivo MRI quantification of LR-NC size is significantly improved by the addition of Gd contrast in individuals with >50% carotid stenosis.

Arteries↗

PubMed2006

Simultaneous outer volume and blood suppression by quadruple inversion-recovery.

A new method has been developed for reduced field-of-view (FOV) imaging with simultaneous blood suppression. This method combines suppression of signals from the outer volume and inflowing blood by using a small-FOV quadruple inversion-recovery (SFQIR) preparative pulse sequence consisting of two double-inversion pulse pairs separated by appropriate delays. Within each pair, inversion pulses are successively applied to the imaged slice and the slab orthogonal to the imaging plane with the thickness equal to the FOV size in the phase-encoding direction. Each double inversion results in the reinversion of the magnetization in the central part of the FOV, while the outer areas of the FOV and inflowing blood remain inverted. The SFQIR module was implemented for single- and multislice acquisition with a fast spin-echo readout sequence. Based on a theoretical model of the signal, the timing parameters of the sequence corresponding to the maximal suppression efficiency can be found by minimizing the variation of the normalized signal over the entire range of T1's that occur in tissues. The method was tested for black-blood imaging of the aorta and carotid arteries, and the results demonstrated its ability to eliminate motion and flow artifacts, reduce scan time, and improve spatial resolution.

Algorithms↗

PubMed2006

MRI of atherosclerosis in clinical trials.

Magnetic resonance imaging (MRI) of the arterial wall has emerged as a viable technology for characterizing atherosclerotic lesions in vivo, especially within carotid arteries and other large vessels. This capability has facilitated the use of carotid MRI in clinical trials to evaluate therapeutic effects on atherosclerotic lesions themselves. MRI is specifically able to characterize three important aspects of the lesion: size, composition and biological activity. Lesion size, expressed as a total wall volume, may be more sensitive than maximal vessel narrowing (stenosis) as a measure of therapeutic effects, as it reflects changes along the entire length of the lesion and accounts for vessel remodeling. Lesion composition (e.g. lipid, fibrous and calcified content) may reflect therapeutic effects that do not alter lesion size or stenosis, but cause a transition from a vulnerable plaque composition to a more stable one. Biological activity, most notably inflammation, is an emerging target for imaging that is thought to destabilize plaque and which may be a systemic marker of vulnerability. The ability of MRI to characterize each of these features in carotid atherosclerotic lesions gives it the potential, under certain circumstances, to replace traditional trials involving large numbers of subjects and hard end-points--heart attacks and strokes--with smaller, shorter trials involving imaging end-points. In this review, the state of the art in MRI of atherosclerosis is presented in terms of hardware, image acquisition protocols and post-processing. Also, the results of validation studies for measuring lesion size, composition and inflammation will be summarized. Finally, the status of several clinical trials involving MRI of atherosclerosis will be reviewed.

Carotid Artery Diseases↗

PubMed2005

Feasibility of in vivo, multicontrast-weighted MR imaging of carotid atherosclerosis for multicenter studies.

PURPOSE: To test the image quality (ImQ) and interscan coverage of MRI for measuring carotid atherosclerosis across multiple centers. MATERIALS AND METHODS: Thirty-nine subjects from five clinical sites (site 1: n=11; site 2: n=16; site 3: n=2; site 4: n=3; site 5: n=7) were imaged on GE 1.5T scanners using a standardized carotid imaging protocol with five weightings (T1, proton density (PD), T2, time-of-flight (TOF), and contrast-enhanced (CE) T1). MR technologists from the five sites received comprehensive protocol training. A maximum coverage of 24 mm (12 slices) was designed for each of four scans (baseline and at four, eight, and 13 weeks). The adequacy of coverage was calculated as the percentage of arteries with at least six slices matched across all four scans. ImQ was evaluated using an established five-point scale for each image. ImQ>or= 3 was considered acceptable for image analysis. RESULTS: Across five sites, the mean ImQ was 3.4-4.2 for T1W, 3.6-4.4 for CE-T1W, 3.4-4.2 for PDW, 3.3-4.2 for T2W, and 3.4-4.0 for TOF. The mean ImQ per site was 3.5-4.2. All sites generated at least six-slice coverage (mean=8.0-9.1) for all index carotid arteries. CONCLUSION: The ImQ and coverage values were comparable among clinical sites using a standardized carotid imaging protocol. With comprehensive protocol training, carotid MRI is technically feasible for use in multicenter studies.

Adult↗

PubMed2005

Sample size calculation for clinical trials using magnetic resonance imaging for the quantitative assessment of carotid atherosclerosis.

PURPOSE: To provide sample size calculation for the quantitative assessment of carotid atherosclerotic plaque using non-invasive magnetic resonance imaging in multi-center clinical trials. METHODS. As part of a broader double-blind randomized trial of an experimental pharmaceutical agent, 20 asymptomatic placebo-control subjects were recruited from 5 clinical sites for a multi-center study. Subjects had 4 scans in 13 weeks on GE 1.5 T scanners, using TOF, T1-/PD-/T2- and contrast-enhanced Tl-weighted images. Measurement variability was assessed by comparing quantitative data from the index carotid artery over the four time points. The wall/outer wall (W/OW) ratio was calculated as wall volume divided by outer wall volume. The percent lipid-rich/necrotic core (%LR/NC) and calcification (%Ca) were measured as a proportion of the vessel wall. For %LR/NC and %Ca, only those subjects that exhibited LR/NC or Ca components were used in the analysis. RESULTS: Measurement error was 5.8% for wall volume, 3.2% for W/OW ratio, 11.1% for %LR/NC volume and 18.6% for %Ca volume. Power analysis based on these values shows that a study with 14 participants in each group could detect a 5% change in W/OW ratio, 10% change in wall volume, and 20% change in %LR/NC volume (power = 80%, p < .05). The calculated measurement errors presume any true biological changes were negligible over the 3 months that subjects received placebo. CONCLUSION: In vivo MRI is capable of quantifying plaque volume and plaque composition, such as %lipid-rich/necrotic core and %calcification, in the clinical setting of a multi-center trial with low inter-scan variability. This study provides the basis for sample size calculation of future MRI trials.

Aged↗

PubMed2004

Hemorrhage in the atherosclerotic carotid plaque: a high-resolution MRI study.

BACKGROUND AND PURPOSE: High-resolution, multicontrast magnetic resonance imaging (MRI) has developed into an effective tool for the identification of carotid atherosclerotic plaque components, such as necrotic core, fibrous matrix, and hemorrhage/thrombus. Factors that may lead to plaque instability are lipid content, thin fibrous cap, and intraplaque hemorrhage. Determining the age of intraplaque hemorrhage can give insight to the history and current condition of the biologically active plaque. The aim of this study was to develop criteria for the identification of the stages of intraplaque hemorrhage using high-resolution MRI. METHODS: Twenty-seven patients, scheduled for carotid endarterectomy (CEA), were imaged on a 1.5-T GE SIGNA scanner (sequences: 3-dimensional time of flight, double-inversion recovery, T1-weighted (T1W), PDW and T2W). Two readers, blinded to histology, reviewed MR images and grouped hemorrhage into fresh, recent, and old categories using a modified cerebral hemorrhage criteria. The CEA specimens were serially sectioned and graded as to presence and stage of hemorrhage. RESULTS: Hemorrhage was histologically identified and staged in 145/189 (77%) of carotid artery plaque locations. MRI detected intraplaque hemorrhage with high sensitivity (90%) but moderate specificity (74%). Moderate agreement in classifying stages occurred between MRI and histology (Cohen kappa=0.7, 95% CI: 0.5 to 0.8 for reviewer 1 and 0.4, 95% CI: 0.2 to 0.6 for reviewer 2), with moderate agreement between the 2 MRI readers (kappa=0.4, 95% CI: 0.3 to 0.6). CONCLUSIONS: Multicontrast MRI can detect and classify carotid intraplaque hemorrhage with high sensitivity and moderate specificity.

Aged↗

PubMed2004

Cross-relaxation imaging reveals detailed anatomy of white matter fiber tracts in the human brain.

Cross-relaxation imaging is a new quantitative MRI modality, which allows mapping of fundamental parameters determining the magnetization transfer (MT) effect in tissues, cross-relaxation rate constant (k) and bound pool fraction (f). This study introduces a new time-efficient technique for cross-relaxation imaging, which obtains three-dimensional (3D) whole-brain k and f maps with scan time of <30 min and isotropic spatial resolution of 1.4 mm. The technical principle of the method is based on four-point fit of a matrix model of pulsed MT to imaging data obtained with variable offset frequency saturation while using a complimentary R1 (=1 / T1) map. Anatomical correlations of in vivo cross-relaxation parametric maps were evaluated in three healthy subjects. The f maps revealed correspondence of areas with highly elevated f = 12-15% to major fiber tracts such as corpus callosum, anterior commissure, optic radiations, and major brain fasciculi. The rest of white matter (WM) demonstrated lower f = 9-11%, resulting in clear visual contrast of fiber tracts. Even lower f = 6.5-8.5% were found in gray matter (GM) with the highest f = 8.5% in the anterior thalamus. Distribution of k was relatively uniform in WM and produced sharp contrast between GM and WM (k = 1.6 and 3.3 s(-1), respectively). The most marked feature of k maps was their ability to visualize the corticospinal tract, which had elevated k = 3.4-3.8 s(-1) but appeared invisible on f maps. The observed patterns on f maps can be explained by variations in the density of myelinated fibers, while the trends of k may reflect regional differences in axonal organization. Cross-relaxation imaging can be used in various clinical studies focused on brain development and white matter diseases.

Analysis of Variance↗

PubMed2003

Multislice double inversion-recovery black-blood imaging with simultaneous slice reinversion.

PURPOSE: To develop a technique for time-efficient multislice double inversion-recovery (DIR) black-blood imaging and to test its applicability and limitations for high-resolution imaging of carotid arteries. MATERIALS AND METHODS: A multislice DIR pulse sequence with fast spin-echo (FSE) readout was implemented on a 1.5 T magnetic resonance (MR) scanner. The principle of the method is that a slice-selective inversion in a DIR preparation reinverts an entire slice pack, and all slices are imaged within repetition time (TR). The number of slices acquired per TR (N) controls the inversion time (TI) to execute the readout for each slice at the zero-crossing point of blood. Multislice DIR images (TR/TE = 2500/9 msec) of carotid arteries were obtained with variable N = 2-8 from four subjects. The method was compared with the standard single-slice DIR and inflow saturation techniques. RESULTS: Multislice DIR with N = 2-6 provided similar flow suppression in carotid arteries as single-slice DIR. At all N = 1-8, blood suppression by DIR was significantly better than by inflow saturation. An additional limitation of multislice DIR was saturation of the signal from stationary tissues that worsened visualization of the vessel wall at N >or= 6. CONCLUSION: Multislice DIR provides up to eight-fold improvement of time-efficiency relative to single-slice DIR and high-quality blood suppression.

Adult↗

PubMed2002

Pulsed Z-spectroscopic imaging of cross-relaxation parameters in tissues for human MRI: theory and clinical applications.

A new method of pulsed Z-spectroscopic imaging is proposed for in vivo visualization and quantification of the parameters describing cross-relaxation between protons with liquid-like and solid-like relaxation properties in tissues. The method is based on analysis of the magnetization transfer (MT) effect as a function of the offset frequency and amplitude of a pulsed off- resonance saturation incorporated in a spoiled gradient-echo MRI pulse sequence. The theoretical concept of the method relies on an approximated analytical model of pulsed MT that provides a simple three-parameter equation for a pulsed steady-state Z-spectrum taken far from resonance. Using this model, the parametric images of cross-relaxation rate constant, content, and T(2) of the semisolid proton fraction can be reconstructed from a series of MT-weighted images and a coregistered T(1) map. The method was implemented on a 0.5 T clinical MRI scanner, and it provided high-quality 3D parametric maps within an acceptable scanning time. The estimates of cross-relaxation parameters in brain tissues were shown to be quantitatively consistent with the literature data. Clinical examples of the parametric images of human brain pathologies (multiple sclerosis and glioma) demonstrated high tissue contrast and clear visualization of the lesions.

Brain↗

PubMed2002

T1-insensitive flow suppression using quadruple inversion-recovery.

A new flow suppression method has been proposed for the acquisition of blood-suppressed (black-blood) images in combination with administration of a positive contrast agent. The technique employs the quadruple inversion-recovery (QIR) preparative pulse sequence, which consists of two double-inversion modules followed by two delays. Within each double inversion, a nonselective RF pulse is immediately followed by a slice-selective one. The time intervals of the sequence can be calculated using an algorithm based on minimization of the variation of a signal equation over an entire range of T(1) occurring in blood before and after contrast administration. QIR is highly insensitive to variations of T(1), providing efficient suppression of a flow signal with T(1) in a range of 200-1200 ms. The technique utilizes identical scan parameters for pre- and postcontrast acquisition, and thus allows reliable quantitative interpretation of contrast enhancement (CE). The clinical application of QIR was demonstrated in high-resolution, contrast-enhanced, black-blood imaging of atherosclerotic plzzaque.

Contrast Media↗